This is a major change to how the main tick interrupt is handled on
the mc1322x platforms. Instead of using two timer resources, TMR0 and
RTC, this patch unifies all the timers to use the RTC. This is enabled by
implementing etimers as scheduled rtimers. The main advantage (aside
from freeing TMR0 for general use) is have the Contiki timebase come
from the same source that will be used for sleeping and wakeup.
